Conventionally, an OLED sequentially includes a substrate, a first electrode layer, an organic layer including an emitting layer and a second electrode layer.
In a structure known as a bottom emitting device, the first electrode layer may be a transparent electrode layer, and the second electrode layer may be a reflective electrode layer. In addition, in a structure known as a top emitting device, the first electrode layer may be formed as a reflective electrode layer, and the second electrode layer may be formed as a transparent electrode layer.
Electrons and holes are injected respectively by the two electrode layers, the injected electrons and holes are recombined in the emitting layer located in the emitting layer, resulting in generating light. The light may be emitted toward the substrate in the bottom emitting device, or toward the second electrode layer in the top emitting device.
In the structure of the OLED, indium tin oxide (ITO) generally used as the transparent electrode layer, the organic layer, and the substrate, which is conventionally formed of glass, have refractive indexes of approximately 2.0, 1.8, and 1.5, respectively. In such a relationship of the refractive index, for example, the light generated in the organic emitting layer in the bottom emitting device is trapped at an interface between the organic layer and the first electrode layer or in the substrate due to a total internal reflection phenomenon, and only a very small amount of light is emitted.